1. Field of the Invention
This invention relates to a raw material supplying device for accurately and stably supplying gas materials such as SiCl.sub.4 or GeCl.sub.4, by controlling the flow rate thereof. The invention can be used in the manufacturing of optical fiber preforms or semiconductor pipes. Also, this invention includes a process for accurately and stably supplying gas material in the manufacturing of optical fibers and semiconductor pipes.
2. Description of the Prior Art
A raw material supplying device for manufacturing optical fiber preforms will be described. A VAD method, OVD method or MCVD method has been known as an optical fiber base material manufacturing method. In the VAD method requiring the continuous supply of gas material for a long period of time, the gas material is formed by gasifying the liquid glass material, and is supplied, under a pressure higher than the atmospheric pressure, to a single or a plurality of multi-layer burners provided for the reaction vessel, so that it is burnt to form a glass powder which is accumulated on the rotating seed rod, thus providing a bar-shaped porous base material (soot boule). The soot boule is dehydrated and consolidated into the preform then spun into an optical fiber. For this purpose, it is necessary to provide a raw material supplying device which can continuously and stably supply a large mount of gas material to the reaction vessel. Also, in the OVD method or MCVD method, a raw material supplying device for continuously supplying gas material to the reaction vessel or quartz pipe is employed. Furthermore, in the forming of semiconductors, a raw material supplying device of this type is employed to supply gas material from a liquid under a pressure higher or lower than the atmospheric pressure.
One example of a known raw material supplying device is shown in FIG. 6. In the raw material supplying device, a liquid material 2 which reacts with the oxygen gas or moisture in the air is placed in a gas-tight raw material tank, and a heater 3 is provided around the raw material tank 1 to gasify the liquid raw material 2. The upper space 4 in the raw material tank 1 is filled with the gas material which is produced by gasification of the li-quid raw material, under the vapor pressure of the raw material determined by the temperature of the raw material tank 1.
A pipe 5 is connected between the upper space 4 of the raw material tank 1 and a reaction vessel (not shown), to supply the gas material to the reaction vessel. The pipe 5 is provided with a valve 6, an automatic valve 7, and a flow control unit 8. A mass flow controller is extensively employed as the flow control unit 8. A pipe 9 with an automatic valve 10 is connected to the pipe 5 between the automatic valve 7 and the flow control unit, to supply nitrogen gas. The automatic valve 10 is operated in association with the automatic valve 7 in such a manner that, when the raw material is being supplied, the automatic valve 7 is open while the automatic valve 10 is closed so that the raw material is sent to the reaction vessel, and, when the raw material is not supplied, the automatic valve 7 is closed while the automatic valve 10 is open so that nitrogen gas is fed into the pipe for protection of the pipe.
With the raw material supplying device of FIG. 6, the gas material is sent into the reaction vessel as follows: When the raw material is not supplied to the reaction vessel, the automatic valve 7 is closed while the automatic valve 10 is open, so that the part of the pipe 5, which is on the reaction vessel side, is purged by the nitrogen gas. In order to start supplying of the raw material, the heater 3 is energized so that the liquid material in the raw material tank 1 is heated and the gas pressure in the upper space 4 of the raw material tank 1 reaches a predetermined value higher than the atmospheric pressure. For instance in the case where the liquid material is SiCl.sub.4 and it is required to increase the gas pressure to about 1 kg/cm.sup.2, heating is carried out until the temperature in the raw material tank reaches 80.degree. C. Under this condition, the automatic valve 10 is closed and the automatic valve 7 is opened, to send the gas material to the reaction vessel. In this operation, the flow control unit 8 operates to control the flow rate of the gas material to a suitable value.
After the supplying of the gas material is completed, the automatic valve 7 is closed and the automatic valve 10 is opened so that the pipe 5 is purged by the nitrogen gas. Of course, where the gas material will not be needed for a time, the heater is deenergized so that the raw material 1 cools down.
Problems arise in the manufacture of optical fibers and semiconductor pipes where the same gas material needs to be supplied to different gas ports at different flow rates or at different times, for example, in the VAD method of manufacturing an optical fiber preform, sometimes a plurality of multi-layer burners is included in one reaction vessel, and it is sometimes required to send the same raw material to the multi-layer burners respectively in different flow rates or with different timing. Furthermore, sometimes it is required that the same raw material is supplied to the ports of one multi-layer burner at flow rates which differ from port to port.
An optical fiber soot boule manufacturing device is shown in FIG. 7 in which the raw material supplying device as shown in FIG. 6 is provided for each port of each of the multi-layer burners. Accordingly, the manufacturing device is unavoidably expensive, with the result that the manufacturing cost is high. In FIG. 7, those parts which have been described with reference to FIG. 6 are designated by the same reference numerals. In the operation of the manufacturing device of FIG. 7, burners 11.sub.1, 11.sub.2 and 11.sub.3 can be separate burners or different ports of a multi-layer burner. In reaction vessel 12, seed rod 14 is rotated, as is known to form a porous glass base material (soot boule), which is gradually pulled upwardly to gradually form optical fiber soot boule 13.
In the case where, in an optical fiber preform manufacturing method, such as the OVD method, MCVD method or VAD method, a plurality of manufacturing devices are operated in a parallel mode, although the same gaseous material is supplied to only one port of each reaction vessel or quartz pipe, it is necessary to use as many of the raw material supplying devices of FIG. 6 for the single gaseous material as there are manufacturing devices, which results in an increase of the manufacturing cost.
Especially in the OVD method, similarly as in the VAD method, sometimes it is necessary to provide the same raw material to one or more ports of each of a plurality of multi-layer burners for one reaction vessel, or to supply the same raw material to the different ports of one multi-layer burner. In these cases, it is necessary to provide a number of raw material supplying devices as shown in FIG. 6, for the same raw material, and therefore the manufacturing apparatus is necessarily expensive.
In view of the foregoing, an object of this invention is to provide a raw material supplying device which is capable of accurately and stably supplying a gas which is used as a raw material for manufacturing an optical fiber preform or a semiconductor from a raw material tank to a plurality of gas burners provided for a reaction vessel or to the ports of a multi-layer burner, in a manner allowing control of the flow rate to each of the gas burners or to each port of a multi-layer burner.
Other objects of the invention will be apparent to the skilled artisan from the detailed description of the invention hereinbelow.